1. Field of the Invention
The present invention relates to a Review Apparatus and to an Inspection System for receiving information on defect locations.
2. Background Art
The recent larger-scale integration and capacity increases of LSI circuits have further reduced the circuit line widths needed for semiconductor devices. Fabrication of semiconductor devices involves the use of photomasks or reticles (hereinafter referred to collectively as masks) on which circuit patterns are formed. The circuit patterns on a mask are photolithographically transferred onto a wafer with the use of a reduced-projection exposure system, often called a stepper, whereby the circuit patterns are formed on the wafer. Masks are manufactured using an electron-beam writing apparatus which is capable of writing fine patterns with an electron beam. Attempts have also been made to develop laser-beam writing apparatuses which use a laser beam for pattern writing. The electron-beam writing apparatus is also used for writing circuit patterns directly onto a wafer.
Yield increase is an important aspect of LSI circuit manufacturing which is considerably costly. However, it will be more difficult to achieve because the sizes of LSI circuit patterns are now on the verge of shifting from submicron to nanometer dimensions, as epitomized by DRAMs (Dynamic Random Access Memory) with a capacity of more than 1 gigabit. One of the major factors leading to yield decrease is defective patterns on masks, but much smaller defective patterns will need to be detected, as necessitated by the size decrease of LSI circuit patterns to be formed on semiconductor wafers. Accordingly, inspection systems for detecting defects on masks need to have a high level of detection accuracy.
One method for defect detection is the die-to-database inspection. Under this method, write data (design pattern data) is input to the inspection system so that the system can create design image data (reference image) from the write data. The write data is often created by converting CAD data into a format a pattern writing apparatus can receive. The system then acquires sensor data (optical image) by imaging patterns, thereby comparing the optical image against the reference image.
In the die-to-database inspection, the light emitted from a light source is directed to a mask via an optical system for inspection of the mask. The mask is placed on a table, and moving the table allows the light to scan across the mask. The light rays that have passed through or been reflected by the mask are focused on an image sensor by a lens, and an optical image acquired by the image sensor is transmitted as sensor data to a comparator. Based on algorithms, the comparator compares the sensor data against its associated design image data. Any discrepancies between the two data sets suggest the presence of defective patterns (see Japanese Patent Laid-Open No. 2008-112178).
A defect inspection system of ten uses multiple algorithms, depending on defect types. Each of the algorithms has a threshold value, and defects are detected when response values exceed the threshold values. To determine a threshold value, a tentative threshold value is first set for an algorithm, and the defect inspection result obtained with the tentative threshold value is then reviewed. These steps are repeated (i.e., another tentative value is set for the algorithm, followed by a review of the result) until sufficient detection sensitivity is obtained. Then, the tentative threshold value that produced the sufficient sensitivity is set as the threshold value of that algorithm.
In defect inspection, possible defects are displayed on a monitor based on the data created from an inspection result, and the operator is supposed to judge whether or not they are real or problematic defects and classify them. Specifically, a comparison image is created from an optical image (sensor data) and a reference image, and the operator reviews possible defects on the comparison image. Each pixel of those images is represented by a gradation value. Specifically, a color palette with 256 gradation values is used to assign to each pixel one value selected from among 0 to 255, whereby write patterns and defects can be displayed.
The optical image is obtained by imaging actually written patterns; thus, the pattern edges (i.e., pattern boundaries) on the optical image often do not look ideal as defined by its associated write data. For instance, even when the write data defines the boundary of a pattern as a rectangle, the optical image of ten displays it as a gentle taper instead of the rectangle. For this reason, gradation values are often subject to gradual changes around pattern edges. Therefore, upon defect judgment, it is necessary to precisely define the locations of pattern edges.
Conventionally, a comparison image is created by calculating the differences in gradation value between a reference image and an optical image. Because any differences between the reference image and optical image are displayed on the comparison image, a careful look at the comparison image allows identification of where defects are located. As stated above, however, much smaller defective patterns will need to be detected, as necessitated by the size decrease of LSI circuit patterns to be formed on semiconductor wafers. Thus, tiny defects on a comparison image will be more difficult to visually identify if the conventional method is used.
Further, although the widths of pattern edges displayed on a reference image and an optical image are defined by a particular value, defects may be hidden by the pattern edges when narrower than the pattern edges. Conventionally, such pattern edges are displayed in black; in such a case, it is extremely difficult to visually identify tiny defects when they are located within the pattern edges. Another problem is that it is more difficult to distinguish between write patterns and pattern edges when the write patterns are extremely small. In that case, tiny defects can be identified more easily by enlarging the gradation value differences between where patterns exist and where they do not, but this also makes large defects less visible.
The present invention has been contrived to address the above problems. That is, one object of the invention is to provide a review apparatus and an inspection system that allow the operator to visually identify defects with ease, irrespective of defect types and sizes.
Other challenges and advantages of the present invention are apparent from the following description.